Bone material is continuously removed by osteoclasts and replaced with new bone material formed by osteoblasts.
This continuous process is highly regulated, i.e. the functioning of osteoclasts and osteoblasts are linked such that total amount of bone is constant e.g. the same amount of bone is formed as is being removed.
Osteoblasts synthesise the collagenous precursors of bone matrix and also regulate its mineralization. The activities of the osteoblasts meet the requirements of skeletal growth and matrix and also regulate its maintenance and mechanical function.
These activities are thought to be influenced by various factors, such as hormones, growth factors, physical activity and other stimuli. Osteoblasts are thought to have receptors for parathyroid hormone and estrogen. Ostoeclasts adhere to the surface of bone undergoing resorption and are thought to be activated by some form of signal from osteoblasts.
Inappropriate regulation of bone re- and degeneration can lead to bone disorders, or metabolic bone diseases. Examples of such diseases are osteoporosis, including post menopausal osteoporosis, Paget's disease and rickets.
Other causes or examples of bone diseases include corticosteroid excess from Cushing's syndrome, hyperthyroidism, hyperparathyroidism, being confined to a bed and bone cancers.
A relative increase in osteoclastic activity, for example, may cause a reduction in bone density and mass, as seen in osteoporosis. Osteoporosis is characterised by a general loss of bone density. Thinning and weakening of the bones leads to increased fracturing from minimal trauma. Osteoporosis may be either a primary disease or secondary to another disease or other diseases.
Osteoporosis is the most common of the metabolic bone diseases and approximately every third women and every 8th man suffers from osteoporosis. Post-menopausal osteoporosis is currently the most common form of osteoporosis. The most prevalent fracturing in post-menopausal osteoporosis is of the wrist and spine. Senile osteoporosis afflicts elderly patients of either sex and younger individuals occasionally suffer from osteoporosis. Senile osteoporosis, is characterised by a higher than average fracturing of the femur.
Oestrogen deficiency has been considered to be a major cause of post-menopausal osteoporosis. Indeed steroids including oestrogen have been used as therapeutic agents supplemented with calcium supplements and bisphosphonates. This treatment resulted in many side-effects, such as weight gain (infiltration of fluid—oedema), nausea, vomiting, headache, bleedings from the uterine (cervix) mucous membrane and increased risk of blood clots. Furthermore the reports of the last years have demonstrated that the oestrogen treatment of postmenopausal osteoporosis have caused breast cancer. Thus the treatment of bone disorders including osteoporosis should preferably be improved to avoid or minimise these side-effects and increase the benefits of treatment.